Nonlinear Spatially Resolved Interferometer for characterizing Optical Properties of Deployed Telecommunication Cables

ABSTRACT

Using pump-probe measurements on multi-span optical links may result in the determination of one or more of the following: 1) wavelength-dependent power profile and gain evolution along the optical link; 2) wavelength-dependent dispersion map; and 3) location of regions of high polarization-dependent loss (PDL) and polarization-mode dispersion (PMD). Such measurements may be a useful diagnostic for maintenance and upgrade activities on deployed cables as well as for commissioning new cables.

TECHNICAL FIELD

This document relates to the technical field of coherent opticalcommunications and more specifically to the characterization of opticalproperties of deployed telecommunication cables, for example, submarinecables.

BACKGROUND

Accurate link budgeting for submarine cables requires detailed knowledgeof the dispersion map and per-wavelength power out of each amplifier(power profile), which may vary from span to span along the link (gainevolution). In practice, complete dispersion and manufacturing data fordeployed cables is often not available to third party terminal equipmentsuppliers or cable owners, and it is only possible to measure the powerprofile at the termination points of the link. Coherent Optical TimeDomain Reflectometers (C-OTDRs) are used to measure loss along a link.

For submarine cables that incorporate High Loss Loop Back (HLLB)channels with filters that reflect a portion of the amplifier output ata specific test channel wavelength into the return path, it is possibleto measure the amplifier output at that test channel wavelength usingC-OTDR. Note that this is only applicable to wavelengths that arereflected into the return path and cannot be used to determine the powerprofile at arbitrary wavelengths.

The present state of the art for link monitoring is summarized in K.Toge and F. Ito, “Recent research and development of optical fibermonitoring in communication systems”, Photonic Sensors, vol. 3, no. 4,pp. 304-313, 2013. The primary diagnostic used on submarine cables isthe coherent OTDR (C-OTDR) which measures loss as a function of distancewithin each span. This technique is very useful for detecting fiberbreaks.

The cumulative dispersion at the end of an optical link is reported bythe WL3 coherent modems commercially available from Ciena Corporationheadquartered in Hanover, Md., USA.

H. Onaka, K. Otsuka, M. Hideyuki and T. Chikama, “Measuring theLongitudinal Distribution of Four-Wave Mixing Efficiency inDispersion-Shifted Fibers”, IEEE Photonics Technology Letters, vol. 6,no. 12, p. 1454, December 1994 describes pump/probe techniques formeasuring the zero dispersion wavelength as well as the nonlinearrefractive index n₂ of an optical fiber (given the effective area of thefiber) by measuring the variation in four wave mixing (FWM) efficiencyas a function of wavelength separation between continuous wave pump andprobe wavelengths. FWM efficiency is maximized at the zero dispersionwavelength and the wavelength dependent periodicity of the FWMefficiency is related to the chromatic dispersion.

Pump/probe techniques for measuring the zero dispersion wavelength havebeen published where the spatial overlap of forward propagating probepulses with backward propagating pump pulses at a different wavelengthis observed through the production of four wave mixing products at intermodulation frequencies. This technique would be difficult to employ onsubmarine cables where counter propagating waves within each span areblocked at each repeater site.

M. Ohashi, “Fiber Measurement Technique Based on OTDR”, “CurrentDevelopments in Optical Fiber Technology”, Dr. Sulaiman Wadi Harun(Ed.), ISBN: 978-953-51-1148-1, reports using the Rayleigh scatteringefficiency to extract the mode field diameter and dispersion. Thesetechniques generally rely on averaging measurement in both directionsand require precise measurement of the back scattered power which willbe difficult to measure when the scattered light has to return to thetransmission site through hundreds of amplifiers.

Most non-destructive polarization-dependent loss (PDL) andpolarization-mode dispersion (PMD) techniques report the valueaccumulated over the length of the optical link. A polarization resolvedOTDR (P-OTDR) which measures the polarization state of theback-scattered light is described in A. Galtarossa and L. Palmieri,“Spatially Resolved PMD Measurements”, Journal of Lightwave Technology,vol. 22, no. 4, p. 1103, 2004. The dynamic range of P-OTDR limits itsreach to several kilometers with a spatial resolution of roughly half ameter. This technique is not applicable to multi-span systems because ofthe difficulty detecting the polarization state of the weakback-scattered signal and disturbance of the scattered lightpolarization state caused by PMD in the return path. P-OTDR is not ableto resolve the circularly polarized component of the birefringencevector as the effect on the forward propagating pulse is canceled whenthe back-scattered light propagates through the same optical path in thereverse direction.

The hinge method described in L. E. Nelson, C. Antonelli, A. Mecozzi, M.Birk, P. Magill, A. Schex, and L. Rapp, “Statistics of Polarizationdependent loss in an installed long-haul WDM system”, Optics Express,vol. 19, no. 7, p. 6790, 2011, can be used to infer the magnitude andnumber of PDL and PMD sections separated by hinges along an opticallink. In this model, PDL activity is assumed to originate from a finitenumber of hinges that are distributed along the fiber, and are separatedby spans with slowly varying PMD. At a given time instant the differentchannels experience the same PDL elements with different polarizationrotations in between them. The PDL measurements at the end of theoptical link for multiple wavelengths are combined into a single PDF. Byfitting to the PDF it is possible to estimate the number of hingeelements and their strengths. A similar technique can be used toidentify PMD hinges.

A simulation technique based on calculating the nonlinear interactionbetween pump and probe pulses propagating at different wavelengths isdescribed in Y. Cao, W. Yan, Z. Tao, L. Li, T. Hoshida and J. Rasmussen,“A fast and accurate method to estimate XPM impact under PMD”, in 10thInternational Conference on Optical Internet (COIN), Yokohama, Japan,2012. They calculate the nonlinear rotation matrix between the pump andprobe pulses at the end of each span. This matrix accounts for theaction of cross-phase modulation (XPM) and cross-polarization modulation(XPolM) in the span. The intent of that paper is to develop a simulationtool. The paper compares their model's predictions to those of splitstep Fourier simulations. There is no discussion of methods for applyingthe ideas of the paper to an experimental measurement.

SUMMARY

Using pump-probe measurements on multi-span optical links may result inthe determination of one or more of the following: 1)wavelength-dependent power profile and gain evolution along the opticallink; 2) wavelength-dependent dispersion map; and 3) location of regionsof high polarization-dependent loss (PDL) and polarization-modedispersion (PMD). Such measurements may be a useful diagnostic formaintenance and upgrade activities on deployed cables as well as forcommissioning new cables.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example bi-directional optical communicationsystem;

FIG. 2 illustrates another example bi-directional optical communicationsystem;

FIG. 3 and FIG. 4 illustrate example monotonic dispersion maps;

FIG. 5 and FIG. 6 illustrate example non-monotonic dispersion maps;

FIG. 7 illustrates an example dispersion-compensated link employingin-line dispersion compensation;

FIG. 8 illustrates an example dispersion-compensated link employingmid-stage access amplifiers;

FIG. 9 illustrates propagation of an example pump pulse, an exampleprobe pulse, and example pilot pulses along an optical link that is adispersion-uncompensated link;

FIG. 10 illustrates propagation of an example pump pulse, an exampleprobe pulse, and an example pilot pulse along an optical link that is adispersion-compensated link, in a system employing high-loss loopbacks;

FIG. 11 illustrates an example pump transmitter and an example probetransmitter in a transceiver;

FIG. 12 illustrates an example of a probe receiver in a transceiver; and

FIG. 13 is a simplified block-diagram illustration of a general-purposecomputer.

DETAILED DESCRIPTION

This disclosure presents methods for using pump-probe measurements onmulti-span optical links. Each optical link comprises multiple spanscoupled by optical amplifiers.

Optical Links

FIG. 1 illustrates an example bi-direction optical communication system2. A first transceiver 4 and a second transceiver 6 are connected via atelecommunications cable (not shown) carrying optical fibers. The cablemay be, for example, a submarine cable or a terrestrial cable.

An optical link 8 from the first transceiver 4 to the second transceiver6 comprises spans 10 of optical fiber coupled by optical amplifiers 12.The optical amplifiers 12 amplify the optical signal. Spans 10 aretypically ˜80 km in length. An optical link 9 from the secondtransceiver 6 to the first transceiver 4 also comprises spans of opticalfiber coupled by optical amplifiers. (For simplicity, only four spans 10and four optical amplifiers 12 are illustrated in each optical link.Typically, the number of spans 10 and the number of optical amplifiers12 in an optical link is much larger.)

FIG. 2 illustrates another example bi-direction optical communicationsystem 3, which differs from the system 2 illustrated in FIG. 1 in thatthe optical link 8 comprises only a single span 10 of optical fiber, andthe optical link 9 comprises only a single span of optical fiber.

In some cases, the optical link 8 is a dispersion-uncompensated link andis characterized by a monotonic dispersion map. FIG. 3 and FIG. 4illustrate example monotonic dispersion maps.

In other cases, the optical link 8 is a dispersion-compensated link andis characterized by a non-monotonic dispersion map. FIG. 5 and FIG. 6illustrate example non-monotonic dispersion maps.

A dispersion-compensated link may employ in-line dispersioncompensation. As illustrated in FIG. 7, each span 10 consists of a firstsegment 14 of optical fiber having a particular dispersioncharacteristic followed by a second segment 16 of optical fiber havingan opposite dispersion characteristic. For example, the optical fiber ofthe first segment 14 may have a positive dispersion and the opticalfiber of the second segment 16 may have a negative dispersion thatcompensates for 90% of the cumulative dispersion in the first segment14. (Such an arrangement may result in the example dispersion mapillustrated in FIG. 5). In another example, the optical fiber of thefirst segment 14 may have a negative dispersion and the optical fiber ofthe second segment 16 may have a positive dispersion that compensatesfor part of the cumulative dispersion in the first segment 14. In afurther example, spans 10 comprised of multiple types of optical fiber(that is, optical fibers having different dispersion characteristics)are used to achieve the dispersion characteristics illustrated in FIG.6.

Alternatively, a dispersion-compensated link may employ mid-stage accessamplifiers. As illustrated in FIG. 8, each span 10 consists of a segmentof optical fiber having a particular dispersion characteristic, and eachoptical amplifier 12 comprises two internal optical amplifiers 18coupled by a segment 20 of optical fiber having an opposite dispersioncharacteristic.

Pump/Probe Interaction Positions

Returning to FIG. 1 and FIG. 2, pump pulses are generated by a pumptransmitter 24 comprised in the first transceiver 4. Probe pulses aregenerated by a probe transmitter 26 at the first transceiver 6. The pumptransmitter 24 and the probe transmitter 26 may be coherenttransmitters, and the probe transmitter 26 may be synchronized to thepump transmitter 24.

The pump pulses and the probe pulses propagate through the optical link8. Optical properties of the probe pulses are measured relative to acoherent reference, and physical properties of the pump pulses arecalculated from the measurements on the probe pulses. The calculatedphysical properties of the pump pulses provide insight into physicalcharacteristics of the optical link 8 and its one or more spans 10.

In appropriate circumstances, a probe pulse is spatially overlapped inthe optical fiber with a pump pulse at one or more positions along theoptical link 8. The pump pulse has a nonlinear interaction with theprobe pulse at each of the one or more positions where the pump pulseand the probe pulse spatially overlap.

For cases where the optical link 8 is a dispersion-uncompensated link,the probe pulse is spatially overlapped with the pump pulse at most oneposition along the optical link 8. In the circumstances where the probepulse is spatially overlapped with the pump pulse at a single positionalong the optical link 8, the calculated physical properties of the pumppulse are associated with that single position, referred to as “theinteraction position” or “the interaction region”.

For cases where the optical link 8 is a dispersion-compensated link,there are circumstances where the probe pulse is spatially overlappedwith the pump pulse at multiple positions along the optical link 8. Inthose circumstances, the pump pulse and the probe pulse will interactover and over again, and it is difficult to determine the nonlinearinteraction at a particular position along the optical link 8 frommeasurements made by a receiver at the end of the optical link 8.Techniques are described below for isolating the nonlinear interactionat a particular position along the optical link 8 or within a particularspan 10. Using those techniques, the calculated physical properties ofthe pump pulse are associated with the particular position or with theparticular span.

In all cases, the spatial resolution at an interaction position dependson the pulse walk-off, which is determined by the dispersion and by thewavelength separation between the pump and the probe wavelengths, and bythe pulse durations. In the limit of long pulses, the pulses may overlapfor the entire length of a span 10. The use of electronic pre-distortionby the pump transmitter 24 to reduce the duration of the pump pulse atan interaction position may decrease the walk-off distance and increasethe spatial resolution.

In one implementation, the probe pulses that are measured havepropagated through the entire optical link 8. This implementation issuitable for cases where the optical link 8 is adispersion-uncompensated link. For example, such measurements may beperformed by a probe receiver 28 at the end of the optical link 8, theprobe receiver 28 comprised in the second transceiver 6. In anotherexample, such measurements may be performed by a probe receiver 30 atthe end of the optical link 9, the probe receiver 30 comprised in thefirst transceiver 4, where the probe pulses are redirected into theoptical link 9 after having propagating through the entire optical link8 and are then propagated over the entire optical link 9 before beingmeasured.

In an alternate implementation, the probe pulses that are measured havepropagated through a portion of the optical link 8. By comparingmeasurements of a probe pulse that has propagated up to a particularspan 10 of the optical link 8 with measurements of a probe pulse thathas propagated through the particular span 10 and not through anysubsequent spans, physical characteristics of the particular span 10 canbe investigated. This implementation is suitable for cases where theoptical link 8 is a dispersion-compensated link and for cases where theoptical link 8 is a dispersion-uncompensated link. For example, opticalfilters (not shown) reflect selected wavelengths into the optical link 9via high loss loopback (HLLB) paths 32, thus creating “replicas” of theprobe pulses that are measured by the probe receiver 30. The HLLBsinstalled at the site of each optical amplifier 12 are used to samplethe probe wavelength as sampled at the output of each optical amplifier12.

The pump transmitter 24 is configured to generate a first modulatedoptical carrier at a pump wavelength λ1 carrying pump pulses. The pumpwavelength λ1 is also referred to as “the pump channel”. In someimplementations, the first modulated optical carrier is generated bymodulating the intensity of an in-service channel at the pump wavelengthλ1 to carry the stream of pump pulses. In other implementations, thefirst modulated optical carrier is unrelated to an in-service channel.The probe transmitter 26 is configured to generate a second modulatedoptical carrier at a probe wavelength λ2 carrying probe pulses. Theprobe wavelength λ2 is also referred to as “the probe channel”. The pumptransmitter 24 and the probe transmitter 26 are configured to transmitthe modulated optical carriers over the optical link 8.

The position or positions at which the probe pulse and the pump pulsespatially overlap depend on the initial time delay Δτ between the pumppulse and the probe pulse. Due to the dispersion in the optical fiberand the wavelength separation Δλ between the pump wavelength λ1 and theprobe wavelength λ2, the pump pulse and the probe pulse propagate in theoptical fiber with different group velocities. The time delay betweenthe pump pulse and the probe pulse, which is initially Δτ at the startof the optical link 8, changes with distance z as z·D Δλ where D is thefiber dispersion parameter. (A non-zero dispersion shifted fiber (NDSF)is a single-mode fiber designed to have a zero-dispersion wavelengthnear 1310 nm. The fiber dispersion parameter D of NDSF is typically ˜17ps/nm/km.) Thus the interaction position z_(int), measured from thestart of the optical link 8, is given by the following formula

$\begin{matrix}{z_{int} = \frac{\Delta\tau}{{\Delta\lambda} \cdot D}} & (1)\end{matrix}$

In some implementations, the pump transmitter 24 and the probetransmitter 26 are electronically synchronized, and the initial timedelay Δτ between the pump pulse and the probe pulse will be known. Inthose implementations, the interaction position z_(int) in the opticallink 8 can be adjusted or controlled by adjusting or varying the initialtime delay Δτ between the pump pulse and the probe pulse, or byadjusting or varying the wavelength separation Δλ. In otherimplementations, the pump transmitter 24 and the probe transmitter 26are not electronically synchronized, and the initial time delay Δτbetween the pump pulse and the probe pulse will be observed.

Coherent Reference—Pilot Pulses

The probe receiver 28,30 measures optical properties of the probe pulserelative to a coherent reference. The coherent reference may be one ormore pilot pulses (or the coherent component thereof, if the one or morepilot pulses are not completely coherent to the probe pulses).

The probe transmitter 26 may be configured so that the second modulatedoptical carrier at the probe wavelength λ2 carries pilot pulses that arearranged to not be spatially overlapped in the optical fiber with any ofthe pump pulses. Stated differently, the second modulated opticalcarrier at the probe wavelength λ2 may carry the probe pulses and thepilot pulses. Thus the probe wavelength λ2 may also be referred to asthe probe/pilot wavelength λ2. The pilot pulses may be designed to havesubstantially similar propagation characteristics to those of the probepulses. Stated differently, the pilot pulses and the probe pulses mayhave nearly identical dispersion, polarization and self-phase modulation(SPM) evolution. Thus changes observed in the probe pulse relative tothe pilot pulse (or to the coherent component of the pilot pulse, if thepilot pulse is not completely coherent to the probe pulse) can beattributed (after suitable averaging) to the nonlinear interactionbetween the pump pulse and the probe pulse. Averaging may include anaveraging over realizations, over polarization and over time sampleswithin the pulse.

For clarity, in the remainder of this document, the term “pilot pulse”refers to the component of the pilot pulse that is coherent to the probepulse.

FIG. 9 illustrates propagation of an example pump pulse 40, an exampleprobe pulse 42, an example advance pilot pulse 44, and an examplefollowing pilot pulse 46 along the optical link 8 at consecutive timesT1, T2, T3 and T4. In this example, the optical link 8 is adispersion-uncompensated link, and the probe pulse 42 is spatiallyoverlapped with the pump pulse 40 at a single interaction position(illustrated at time T3). The single interaction position can beadjusted by controlling the initial time delay AT between the pump pulse40 and the probe pulse 42, as discussed above. The advance pilot pulse44 and the following pilot pulse 46 are located on either side of theprobe pulse 42, and may be used to estimate and correct differencesbetween transmit and receive laser sources.

FIG. 10 illustrates propagation of an example pump pulse 50, an exampleprobe pulse 52, and an example pilot pulse 54 along the optical link 8at consecutive times T1, T2, T3 and T4. In this example, the opticallink 8 is a dispersion-compensated link, and the probe pulse 52 isspatially overlapped with the pump pulse 50 at multiple interactionpositions (illustrated at time T2, time T3 and time T4). The interactionpositions can be adjusted by controlling the initial time delay ATbetween the pump pulse 50 and the probe pulse 52, as discussed above.

In this example, optical filters (not shown) are installed at the siteof each optical amplifier 12 to reflect a portion of the probe pulse 52and a portion of the pilot pulse 54 into the optical link 9 via the HLLBpaths 32. The reflected portion of the probe pulse 50 is referred to asa “probe replica” and the reflected portion of the pilot pulse 54 isreferred to as a “pilot replica”. The pump wavelength λ1 of the firstmodulated optical carrier, which carries the pump pulses, is chosen tobe outside of the bandwidth of the optical filter, so that the pumppulse 50 is not reflected into the optical link 9 and does not interactwith any of the replicas on the return path. Thus the HLLBs are able toisolate the interaction between the pump pulse 50 and the probe pulse 54within a given span 10.

In this example, a first replica 512 of the probe pulse 52 (“first probereplica 512”) and a first replica 514 of the pilot pulse 54 (“firstpilot replica 514”) are shown at time T2, having traversed the HLLB path32 at the first optical amplifier 12. Because the pump pulse 50 and theprobe pulse 52 do not interact along the span 10 between the firsttransceiver 4 and the first optical amplifier 12, the first probereplica 512 and the first pilot replica 514 are nearly identical to eachother.

A second replica 522 of the probe pulse 52 (“second probe replica 522”)and a second replica 524 of the pilot pulse 54 (“second pilot replica524”) are shown at time T3, having traversed the HLLB path 32 at thesecond optical amplifier 12. Because the pump pulse 50 and the probepulse 52 do interact along the span 10 between the first opticalamplifier 12 and the second optical amplifier 12, the second probereplica 522 differs from the second pilot replica 524 in phase andpolarization state. Measurements (performed by the receiver 30) of thedifferences are indicative of cumulative effects of nonlinearinteractions between the pump pulse 50 and the probe pulse 52 along theoptical link 8 up to the second optical amplifier 12.

A third replica 532 of the probe pulse 52 (“third probe replica 532”)and a third replica 534 of the pilot pulse 54 (“third pilot replica534”) are shown at time T4, having traversed the HLLB path 32 at thethird optical amplifier 12. Because the pump pulse 50 and the probepulse 52 do interact along the span 10 between the second opticalamplifier 12 and the third optical amplifier 12, the third probe replica532 differs from the third pilot replica 534 in phase and polarizationstate. Measurements (performed by the receiver 30) of the differencesare indicative of cumulative effects of nonlinear interactions betweenthe pump pulse 50 and the probe pulse 52 along the optical link 8 up tothe third optical amplifier 12. Comparing the results up to the secondoptical amplifier with the results up to the third optical amplifierallows extraction of properties of the span 10 between the secondoptical amplifier 12 and the third optical amplifier 12.

Measured Optical Properties

As mentioned above, changes observed in the probe pulse relative to thepilot pulse can be attributed (with suitable averaging) to the nonlinearinteraction between the pump pulse and the probe pulse. The measuredoptical properties of the probe pulse may include a common mode phase ofa probe pulse relative to a common mode phase of a pilot pulse. Themeasured optical properties of the probe pulse may include apolarization state of a probe pulse relative to a polarization state ofa pilot pulse.

Pilot pulses may further be used to estimate the phase differencebetween a pilot pulse and a probe pulse that results from differencesbetween the transmit and receive lasers. In one implementation, anadvance pilot pulse and a following pilot pulse may be located on eitherside of a given probe pulse. Other configurations of the pilot pulsesare possible but are not described for brevity. The common mode phaseshift between the advance pilot pulse and the following pilot pulse isattributed to the differences. The differences may include residualintermodulation frequency (IF). Residual IF contributes an error to thecross-phase modulation (XPM) phase shift measured between the probepulse and one of the pilot pulses. This error may be corrected bysubtracting the phase shift measured between the two pilot pulses scaledby the ratio of the time delay between the two pilot pulses to the timedelay between the chosen pilot and probe pulses, from the measured XPMphase shift. This is described in more detail below with respect toEquation 13.

Wavelength Dependence

In a pump/probe experiment, the pump pulse propagates at the pumpwavelength λ1. The calculated physical properties of the pump pulsecorrespond to the nonlinear interaction that took place in the one ormore interaction regions at the pump wavelength λ1. A property of thepump pulse may be calculated for a range of wavelengths whereexperiments are repeated at each of the pump wavelengths of interest, bygenerating pump pulses on a modulated optical carrier at a given pumpwavelength λ1 and measuring the nonlinear interaction between the pumppulse and the probe pulse in the one or more interaction regions.

In one instantiation the probe wavelength λ2 is maintained at a constantvalue while experiments are repeated for each of the chosen pumpwavelengths λ1. This configuration would be most relevant when usingHLLBs to extract the probe pulse, as the probe wavelength λ2 would beconfined to one of the wavelengths reflected into the return path(optical link 9) by the HLLBs.

In another instantiation when used to probe an optical link 8characterized by a monotonic dispersion map, the wavelength separationΔλ between the first modulated optical carrier (which carries the pumppulse) and the second modulated optical carrier (which carries the probepulse and optionally, the pilot pulse) can be held constant, and thewavelengths of the pump and probe channels can move together todetermine the dependence of the measure properties on pump wavelength.

Theory of XPM and XPolM

The nonlinear interaction between modulated optical carriers propagatingon a fiber optic link is well understood by those familiar with the artand is described in detail in the scientific literature. In short, thenonlinear interaction between a pump pulse propagating at the pumpwavelength ⊇1 and a probe pulse propagating at the probe wavelength λ2is dominated by the processes of cross-phase modulation (XPM) andcross-polarization modulation (XPolM).

The pump pulses, the probe pulses, and the pilot pulses may be describedby complex optical field Jones vectors |A(z,t)

, |p(z,t)

, and |r(z, t)

respectively, where z represents the distance along the optical link 8and t represents the time since initial launch into the optical link 8.These can be represented as three-dimensional Stokes vectors {rightarrow over (A)}(z, t) for the pump pulse, {right arrow over (p)}(z,t)for the probe pulse, and {right arrow over (r)}(z, t) for the pilotpulse, where the transformation from Jones space to Stokes space iscalculated using the Pauli spin matrices, {right arrow over (σ)}=[σ₁,σ₂, σ₃], as {right arrow over (A)}(z,t)=

A(z,t)|{right arrow over (σ)}|A(z,t)

, {right arrow over (p)}(z,t)=

p(z,t)|{right arrow over (σ)}|p(z,t)

, and {right arrow over (r)}(z,t)=

r(z,t)|{right arrow over (σ)}|r(z,t)

.

As a result of nonlinear interaction with the pump pulse, thepolarization state and phase of the probe pulse may be modified. Theprobe Jones vector following the interaction may be approximated as:

|p(z,t)

=e ^(−iϕ) ^(XPM) ^((z,t)) U _(XPolM)(z,t)|r(z,t)

  (2)

where the common mode phase ϕ_(XPM)(z,t) and the 2×2 matrix U_(XPolM)(z,t) describe the XPM and XPolM imparted on the probe channel,respectively.

A simplified model for the XPM and XPolM activity on the probe inducedby the pump is given subject to the following simplifying assumptions(referred to below as “the simplifying assumptions”):

-   -   a. The power of the pump pulse is much stronger than that of the        probe such that propagation with the probe pulse does not        significantly modify the properties of the pump pulse.    -   b. The pump pulse is assumed to propagate linearly such that its        propagation model only considers contributions from dispersion,        attenuation and gain. The additional variations of the pump        polarization state caused by XPolM interactions between the pump        pulse and interfering WDM channels average to zero, provided the        interfering WDM channels have no net degree of polarization.        Further, any additional phase that is common to both the pilot        pulses and the probe pulses does not impact the estimate of        ϕ_(XPM)(z, t).    -   c. The channel separation Ω is large compared to the pump and        probe/pilot channel bandwidth such that intra-channel dispersion        can be neglected.    -   d. The channel bandwidth is small such that intra-channel        polarization-mode dispersion (PMD) is negligible.    -   e. The efficiency for non-degenerate four wave mixing (FWM) is        small compared to that of either XPM or XPolM such that FWM        effects can be ignored.    -   f. The polarization transfer function is evaluated to        leading-order in the Magnus series.

Consider the spans 10 of the optical link 8 to be indexed by the index nhaving integer values from 1 to N, where N is the total number of spans10 in the optical link 8. Under the above simplifications the XPM phaseshift induced in the probe pulse by the pump pulse following propagationthrough spans 1 to N is:

$\begin{matrix}{{{\varphi_{XPM}\left( {N,t} \right)} = {\frac{3}{2}{\int_{- \infty}^{\infty}{\frac{d\; \omega}{2\pi}e^{{+ i}\; \omega \; t}{\sum\limits_{n = 1}^{N}{{\exp \left( {\sum\limits_{k = 1}^{n - 1}{{- i}\; \omega \; d_{k}L_{k}}} \right)}{P_{n,{mean}} \cdot {H_{XPM}\left( {n,\omega} \right)}}}}}}}}\mspace{20mu} {{where},}} & (3) \\{\mspace{79mu} {{H_{XPM}\left( {n,\omega} \right)} = {\gamma_{n}{\frac{\left( {1 - {\exp \left( {{- \left( {\alpha_{n} + {i\; d_{n}\omega}} \right)}L_{n}} \right)}} \right)}{\left( {\alpha_{n} + {i\mspace{11mu} d_{n}\omega}} \right)} \cdot \frac{{\overset{\sim}{P}}_{1}(\omega)}{P_{1,{mean}}}}}}} & (4)\end{matrix}$

is an effective XPM transfer function for propagation through span n,α_(n) is a fiber attenuation constant for span n, γ_(n) is a nonlinearparameter for span n , L_(n) is the length of span n, and d_(n) is thedispersive walk-off between the pump pulse and the probe pulse in spann. In the above expressions P_(n,mean) is the mean pump power at theinput to span n which is defined as:

$P_{n,{mean}} = {\frac{1}{T}{\int_{0}^{T}{{dt}\mspace{14mu} E\left\{ {P_{n}(t)} \right\}}}}$

where T is a time interval characteristic of the pump pulse andE{P_(n)(t)} is an expectation of the instantaneous power over anystochastic variations. The instantaneous power P_(n)(t)=

A_(n)(t)|A_(n)(t)

is the power of the pump pulse at the input to span n and {umlaut over(P)}_(n)(ω) is the time-to-frequency Fourier transform of P_(n)(t). TheXPM transfer function (Equation 4) is shown referenced to the powerspectrum in the first span. We note that any other span could be used.

The nonlinear constant γ_(n) for span n depends on the effective areaA_(eff) of the optical fiber, the pump wavelength λ1, and the nonlinearrefractive index n2, as:

$\begin{matrix}{\gamma_{n} = {\frac{8}{9} \cdot \frac{2\pi \; n_{2}}{\lambda_{1}A_{eff}}}} & (5)\end{matrix}$

Through XPolM, the polarization Stokes vector of the probe pulseprecesses around the Stokes vector of the pump pulse. Using thesimplifying assumptions, the pump pulse is assumed to propagate linearlyand is described by the position-dependent Stokes vector:

$\begin{matrix}{{\overset{\sim}{A}\left( {z,\omega} \right)} = {{P(z)}{\exp \left( {- {\int_{0}^{z}{{dz}^{\prime}i\; \omega \; {d\left( z^{\prime} \right)}}}} \right)}{{T_{m}(z)} \cdot \frac{\overset{\sim}{A}\left( {0,\omega} \right)}{P(0)}}}} & (6)\end{matrix}$

where T_(m)(z) is the 3×3 rotation matrix that describes the rotation ofthe pump polarization state while the pump pulse is propagating todistance z and P(z) is the average pump power at distance z. TheXPolM-induced polarization rotation U_(XPolM)(z, t) is given by:

$\begin{matrix}{{U_{XPolM}\left( {z,t} \right)} = {{xp}\left\lbrack {{- \frac{i}{2}}{{{\overset{\rightarrow}{r}}_{XPolM}\left( {z,t} \right)} \cdot \overset{\rightarrow}{\sigma}}} \right\rbrack}} & (7)\end{matrix}$

where {right arrow over (r)}_(XPolM)(z, t) represents a time-dependent3×1 XPolM rotation vector:

$\begin{matrix}{{{r_{XPolM}\left( {z,t} \right)} = {\int_{- \infty}^{\infty}{\frac{d\; \omega}{2\pi}e^{i\; \omega \; t}{F_{m}\left( {z^{\prime},\omega} \right)}\frac{\overset{\sim}{A}\left( {0,\omega} \right)}{P(0)}}}}{where}} & (8) \\{{F_{m}\left( {z,\omega} \right)} = {\int_{0}^{z}{{dz}^{\prime}{\gamma \left( z^{\prime} \right)}{P\left( z^{\prime} \right)}{\exp \left( {- {\int_{0}^{z^{\prime}}{{dz}^{''}i\; \omega \; {d_{m}\left( z^{''} \right)}}}} \right)}{T_{m}\left( z^{\prime} \right)}}}} & (9)\end{matrix}$

is an XPolM transfer function.

If the pump pulse and the probe pulse interact over a finite distancefrom position z₁ to position z₂ (or if we sample the probe pulse atpositions z₁ and z₂), U_(ZPolM) will be related to the pump polarizationstate at z₁ modified by its evolution while propagating from z₁ to z₂.As T_(m)(z) is not generally known we can estimate the expectation valuefor the pump polarization rotation matrix for propagation from z₁ to z₂as

T_(m)(z₂; z₁)

=e^(−η) ^(m) ^((z) ² ^(−z) ¹ ⁾I₃ where η_(m)=Ω_(m) ²D_(pmd) ²/3 is a PMDdiffusion parameter and I₃ is the 3×3 identity matrix. The pump/probetechnique may be used to characterize the pump polarization state at z₁where polarization state diffusion from z₁ to z₂ introduces a boundedmeasurement error.

A pump pulse that enters a given span n at position z₁ and interactswith the probe pulse from position z₁ to position z₂ will induce XPoMactivity in the probe pulse which may be observed by sampling the probepulse at the entrance to span n+1. Averaging over diffusion of the pumppolarization state during propagation, the XPolM rotation vector is:

$\begin{matrix}{{\langle{{\overset{\rightarrow}{r}}_{XPolM}\left( {{z_{2};z_{1}},t} \right)}\rangle} = {\gamma {\int_{- \infty}^{\infty}{\frac{\left( {1 - {\exp \left( {{- \left( {\alpha_{n} + {i\mspace{14mu} d_{n}\omega} + \eta_{n}} \right)}\left( {z_{2} - z_{1}} \right)} \right)}} \right.}{\left( {\alpha_{n} + {id}_{n} + \eta_{n}} \right)}\frac{\overset{\sim}{A}\left( {z_{1},\omega} \right)}{P\left( z_{1} \right)}}}}} & (10)\end{matrix}$

where Ã(z₁, ω) is the polarization Stokes vector of the pump pulse atposition z₁ and γ is taken as constant between z₁ and z₂.

For cases where the pump pulse overlaps with the probe pulse for thefull propagation through a span n, the transformation matrix R thatrelates the probe pulse's Jones vector at the start of the span n to theprobe pulse's Jones vector at the output of the span n may beapproximated as:

$\begin{matrix}{{R = {{\exp \left( {{- \frac{3}{2}}\gamma_{n}L_{eff}P_{\max}} \right)}{\exp \left( {{- \frac{i}{2}}\gamma_{n}L_{eff}P_{\max}{\hat{s} \cdot \overset{\rightarrow}{\sigma}}} \right)}}}{where}} & (11) \\{L_{eff} = \frac{1 - {\exp \left( {{- \alpha_{n}}L} \right)}}{\alpha_{n}}} & (12)\end{matrix}$

is the effective interaction distance, ŝ is the pump polarization Stokesvector at the input to the span n, P_(max) is the maximum pump power atthe input to the span n, and polarization diffusion effects have beenignored.

Pump/Probe/Pilot (3P) Trials

In a pump/probe/pilot (3p) trial, pump, probe and pilot pulses arelaunched into the optical link 8 such that the pump pulse and the probepulse may interact at one or more interaction position(s). For eachtrial, the pump and probe/pilot pulses may be prepared in differentstates where properties such as the relative time delay, dispersion,amplitude, duration, shape or polarization state are varied.

Repeated measurements with varying pump-probe configurations enable oneor more of the following properties of a bi-directional opticalcommunication system 2,3 to be determined: 1) wavelength-dependent powerprofile and gain evolution along the optical link 8; 2)wavelength-dependent dispersion map; and 3) location of regions of highpolarization-dependent loss (PDL) and polarization-mode dispersion(PMD). These properties of the optical link 8 are determined as afunction of optical path length which may be related to distance alongthe optical link 8, and it may be possible to determine these propertiesfor different positions within the same span 10.

To measure properties of the nonlinear interaction that took place at agiven interaction point, one or more 3p trials are conducted for thepump and probe/pilot prepared in one or more states labeled withsubscripts k as |A_(k)(z,t)

, |p_(k)(z,t)

and |r_(k)(z,t)

.

MEASUREMENT EXAMPLES

Determination of Power Profile/Tilt

The power of the pump pulse in the interaction region may be inferredfrom the XPM phase shift induced in the probe pulse. Due to XPM, thepower envelope of the pump pulse is imprinted into the phase of theprobe pulse. The magnitude of the common mode phase shift imprinted inthe probe pulse by the pump pulse is one measure of the strength of thenonlinear interaction between the pump pulse and the probe pulse. Themagnitude of the common mode phase shift imprinted in the probe pulse bythe pump pulse is measured at the receiver relative to a coherentreference, and is proportional to the power of the pump pulse at theinteraction position.

Following propagation the probe/pilot wavelength λ2 is sampled at theoptical amplifier at the end of span n. The probe/pilot wavelength λ2may be sampled at the end of the optical link 8. The XPM phase shift isextracted from the detected probe pulses and pilot pulses as:

$\begin{matrix}{{\varphi_{XPM}(n)} = {\angle \; E\left\{ {\frac{1}{t_{2} - t_{1}}{\int_{t_{1}}^{t_{2}}\left. \langle\ {{p(t)}{{r\left( {t + {\Delta \; T}} \right)}\rangle}{dt}} \right\}}} \right\}}} & (13)\end{matrix}$

where the integration is typically over a region near the peak of thepulse, ΔT is the temporal delay between probe and pilot pulses, and E{.. . } denotes the expectation over repeated 3p trial instances andpolarization states. When a second pilot pulse is used for IF estimationwhere the first pilot pulse precedes the probe by ΔT₁ and the secondpilot follows the probe by ΔT₂ the laser phase ramp is given by:

${\varphi_{L}(n)} = {\angle \; E\left\{ {\frac{1}{\left( {t_{2} - t_{1}} \right)\left( {{\Delta \; T_{1}} + {\Delta \; T_{2}}} \right)}{\int_{t_{1}}^{t_{2}}\left. \langle{{r\left( {t - {\Delta \; T_{1}}} \right)}{{r\left( {t + {\Delta \; T_{2}}} \right)}\rangle}{dt}} \right\}}} \right\}}$

and the corrected XPM phase is:

${\varphi_{XPM}(n)} = {{\angle \; E\left\{ {\frac{1}{t_{2} - t_{1}}{\int_{t_{1}}^{t_{2}}\left. \langle{{r\left( {t - {\Delta \; T_{1}}} \right)}{{p(t)}\rangle}{dt}} \right\}}} \right\}} - {{\varphi_{L}(n)}\Delta \; T_{1}} + {\angle \; E\left\{ {\frac{1}{t_{2} - t_{1}}{\int_{t_{1}}^{t_{2}}\left. \langle\left. {p(t)} \middle| {{r\left( {t + {\Delta \; T_{2}}}\rangle \right.}{dt}} \right. \right\}}} \right\}} - {{\varphi_{L}(n)}\Delta \; T_{2}}}$

where E{. . . } denotes the expectation over repeated 3p trial instancesand polarization states. The XPM phase shift is related to the pumppower in the interaction region(s) for example by Equation (3). Themeasured phase shift ϕ_(XPM)(n) includes the cumulative XPM phase shiftfrom all pump/probe interactions up to the output of the opticalamplifier at the end of span n.

For the case where the pump pulse and the probe pulse overlap for thefull length of span n and the probe XPM phase shift is sampled at theinput to span n as well as the input to span n+1 (such as in the casewhere HLLBs are used), the pump power in span n may be calculated using:

ϕ_(XPM)(n+1)−ϕ_(XPM)(n)=− 3/2γ_(n) L _(eff) P _(max)  (14)

The polarization averaging may be satisfied by repeating over a seriesof trials with the pump pulse prepared in appropriate polarizationstates, by repeating over a series of trials with the probe pulseprepared in appropriate polarization states, by repeating over a seriesof trials with the pump pulse and the probe pulse prepared inappropriate polarization states, by randomizing the pump polarization orthe probe polarization or both at the transmitter site, or by operatingunder conditions of sufficient PMD such that polarization averagingoccurs during propagation.

The XPM phase shift at a given position may also be measured byadjusting the initial time delay Δτ between the pump pulse and the probepulse in order to position the interaction region at the chosenposition, and then measuring ϕ_(XPM)(n) at a span n located after theinteraction region. In that case the XPM phase shift is again related tothe pump power in the interaction region for example through Equation(3). The mapping from phase shift to power is calculated with thetransmit pulse shapes and pump/probe wavelength separation asparameters.

By adjusting or varying the pump wavelength λ1 of the first modulatedoptical carrier that carries the pump pulses, the wavelength variationof the optical power (power tilt) at the overlap position(s) can bemeasured, and by varying the overlap position(s), the power tilt can bemapped as a function of distance z from the start of the optical link 8.Stated differently, by performing repeated transmissions of pump pulsesand probe pulses with different wavelengths λ1 for the pump pulse, andthen measuring the phase of the corresponding probe pulse, the change inthe power of the pump pulse at a given distance z from the start of theoptical link 8 is mapped as a function of wavelength.

If the fiber type and parameters γ and α are not known, common modephase shifts can be used to determine the relative variation in power ofthe pump pulse as a function of wavelength. For example, a first commonmode phase shift ϕ1 determined from calculation using ϕ_(XPM) (forexample from Equation (3)) on measurements performed with the pump pulsecarried by a modulated optical carrier at a first pump wavelength λ1 isproportional to the power P1 of the pump pulse at the interactionposition within the particular span. A second common mode phase shift ϕ2determined from measurements performed with the pump pulse carried by amodulated optical carrier at a second pump wavelength λ1 is proportionalto the power P2 of the pump pulse at the interaction position within theparticular span. Thus the ratio of the second common mode phase shift ϕ2to the first common mode phase shift ϕ1 is equivalent to a ratio of thepower P2 to the power P1, and provides a measure of the relativevariation in power of the pump pulse at the interaction point within theparticular span as a function of pump wavelength λ1 of the firstmodulated optical carrier, which carries the pump pulses. Measurementsof the power profile are of interest in submarine applications where thelink can develop substantial power tilts.

Calculation of Dispersion Map by determining Strength of Non-LinearInteraction

As mentioned above, HLLBs may be used to sample the probe field at theoutput of each span. The nonlinear interaction between the pump pulseand the probe pulse within a given span is maximized when the pump pulseand the probe pulse spatially overlap within the span. The strength ofthe nonlinear interaction can be observed through the magnitude of thecommon mode phase shift imprinted in the probe pulse by the pump pulsedue to XPM. By comparing the common mode phase of a probe replicareflected immediately prior to a particular span of the optical link 8to the common mode phase of a probe replica reflected immediately aftertraversing the particular span, one can determine the strength of thenonlinear interaction between the pump pulse and the probe pulse in theparticular span. By varying the initial time delay Δτ between the pumppulse and the probe pulse, one can determine the initial time delay Δτthat maximizes the nonlinear interaction between the pump pulse and theprobe pulse within a particular span of the optical link 8. Whencombined with the known wavelength separation Δλ between the carriers ofthe pump pulse and the probe pulse, the initial time delay Δτ thatmaximizes the nonlinear interaction within a particular span can be usedto calculate the cumulative dispersion at each optical amplifier 12 andthe dispersion map for the optical link 8.

In yet another instantiation the dispersion can be measured on anoptical link without employing HLLBs by first measuring the non-linearinteraction strength as a function of pump/probe initial time delay Δτ.The locations of optical amplifier sites will be apparent in the recordof the dependence of the non-linear interaction strength on the initialpump/probe time delay Δτ manifesting as a rapid increase in interactionstrength at the position of an optical amplifier followed by anexponential decrease as the overlap position moves into a given span.The initial time delay Δτ combined with the pump/probe wavelengthseparation Δλ gives the cumulative dispersion at each optical amplifiersite. The distance between optical amplifier sites can be measured witha conventional OTDR which, when combined with the cumulative dispersionat each optical amplifier site, gives the dispersion in each span.

Determination of Pump Polarization State, PMD and PDL

In another instantiation the pump polarization state may be inferredfrom the change in probe polarization state following the nonlinearinteraction with the pump. Due to XPolM, the polarization state of theprobe pulse is modified by the polarized pump pulse. More specifically,the polarization state of the probe pulse will precess around an axisdefined by the polarization Stokes vector of the pump pulse. Observingthe polarization state of the probe pulse at the receiver (for a rangeof initial polarization states of the probe pulse) enables theextraction of the polarization state of the pump pulse at theinteraction position.

In order to uniquely define the pump polarization state, in thereference frame of the probe pulse, trials are conducted with the probepulse prepared in a minimum of two linearly independent polarizationstates. Measurements for trials with the probe pulse prepared in thedifferent polarization states are related through the 3×3 complextransformation matrix R as:

R[{right arrow over (r ₁)}(z, t+ΔT), {right arrow over (r ₂)}(z, t+ΔT),. . . ]=[{right arrow over (p₁)}(z,t), {right arrow over (p ₂)}(z,t), .. . ]  (15)

where ΔT is the temporal delay between probe and pilot pulses, and{right arrow over (p_(k))}(z, t)=

p_(k)(z,t)|{right arrow over (σ)}|p_(k)(z,t)

and {right arrow over (r_(k))}(z,t)=

r_(k)(z,t)|{right arrow over (σ)}|r_(k)(z,t)

are the Stokes space representations of the probe fields and pilotfields respectively.

The transformation matrix R which is common to the linearly independenttrials uniquely determines the polarization state of the pump pulse atthe interaction position. The solution for the transformation matrix Rrequires the polarization state of the pump pulse to be static relativeto the reference frame of the probe pulse for the duration of thetrials. The typical polarization temporal autocorrelation time forsubmarine cables is on the order of hours to days, whereas the durationof the trials is expected to be on the order of several seconds.

During the first trial, the Stokes vectors {right arrow over (r₁)} (z,t+ΔT) of the pilot pulse and {right arrow over (p₁)}(z, t) of the probepulse are measured. During the second trial, the Stokes vectors {rightarrow over (r₂)}(z, t+ΔT) of the pilot pulse and {right arrow over(p₂)}(z, t) of the probe pulse are measured. The measurements continuefor the set of pump/probe/pilot configurations.

A matrix P is formed of the Jones vectors of the probe pulse, as

P=[{right arrow over (p₁)}(z,t), {right arrow over (p ₂)}(z,t), . . .]  (16)

A matrix M is formed of the Jones vectors of the pilot pulse, as

M=[{right arrow over (r ₁)}(z, t+ΔT), {right arrow over (r ₂)}(z, t+ΔT),. . . ]  (17)

The matrix PM^(\), which is composed of the matrix P and the Hermitianadjoint (conjugate transpose) of the matrix M, can be factored usingsingular value decomposition (SVD) as

PM ^(\) =UΣV ^(\)  (18)

where U and V are unitary matrices and Σ is a diagonal matrix ofsingular values. The transformation matrix R is then given by UV^(\),that is:

R=UV^(\)  (19)

The Stokes vector describing the polarization state of the pump pulse,{right arrow over (v)}=[v₁, v₂, v₃]^(T) at the interaction region isrelated to R by:

$\begin{matrix}{{{R - R^{\dagger}} = {2\mspace{14mu} {\sin (\varphi)}\begin{pmatrix}0 & {- v_{3}} & v_{2} \\v_{3} & 0 & {- v_{1}} \\{- v_{2}} & v_{1} & 0\end{pmatrix}}}{and}} & (20) \\{\hat{v} = \frac{\overset{\rightarrow}{v}}{\overset{\rightarrow}{v}}} & (21)\end{matrix}$

The normalized Stokes vector p can be determined without knowledge ofthe normalization constant sin (ϕ).

The evolution of the pump polarization state may be tracked as the pumppulse propagates through the optical link 8 by comparing the change inpolarization state between successive probe replicas (relative to pilotreplicas) when performing measurements with HLLBs.

Within a single span, or along a multi-span optical link characterizedby a monotonic dispersion map, the evolution of the pump polarizationstate may be mapped by adjusting or varying the initial time delay Δτbetween the pump pulse and the probe pulse. Stated differently, byperforming repeated transmissions of pump pulses and probe pulses withdifferent initial time delays between a pump pulse and correspondingprobe pulse, and then measuring the polarization state of thecorresponding probe pulse, the change in the polarization state of thepump pulse as the pump pulse propagates down the optical link 8 ismapped as a function of distance either between replicas when usingHLLBs or from the start of the optical link 8 when measuring a span 10characterized by a monotonic dispersion map.

The change in the polarization state of the pump pulse can be measuredas a function of distance (interaction point) and as a function ofwavelength separation between the pump pulse and the probe pulse. Thesemeasurements characterize the polarization-mode dispersion (PMD) as afunction of distance along the optical link 8.

Determination of the pump polarization state may be repeated for asequence of pump pulses prepared in non-collinear polarization states.By performing repeated transmissions of pump pulses and probe pulseswith different initial polarization states for the pump pulse, it ispossible to identify the polarization state of the pump pulse (within agiven interaction region) that maximizes the common mode phase of theprobe pulse and the polarization state of the pump pulse (within thesame interaction region) that minimizes the common mode phase of theprobe pulse. Comparing the common mode phase between the best case andthe worst case of polarization orientations gives a measure of themagnitude and orientation of the polarization-dependent loss (PDL). Thusthe location of regions of high polarization-dependent loss (PDL) can bedetermined.

Transmitter Design, Receiver Design and Dsp

FIG. 11 illustrates an example of the pump transmitter 24 and an exampleof the probe transmitter 26 in the first transceiver 4. The pumptransmitter 24 and the probe transmitter 26 both generally comprise anelectronic waveform synthesizer 34 to generate a waveform signal 36. Thewaveform signal 36 is then supplied to a modulator 38 for modulatingrespective dimensions of a continuous wave (CW) optical carrier inaccordance with the waveform signal 36. In the case of the pumptransmitter 24, the CW optical carrier is at the pump wavelength λ1. Inthe case of the probe transmitter 26, the CW optical carrier is at theprobe wavelength λ2. The CW optical carrier is typically generated by alaser 40 in a manner known in the art, and the modulator 38 may beimplemented using any of a variety of known modulator devices, such asphase modulators, variable optical attenuators, Mach-Zehnderinterferometers, etc. A multiplexer 42 combines the modulated opticalsignal appearing at the output of the modulator 38 of the pumptransmitter 24 with the modulated optical signal appearing at the outputof the modulator 38 of the probe transmitter 26, and optionally, withother traffic-carrying channels 44, and the combined signals aretransmitted through the optical link 8.

FIG. 12 illustrates an example of the probe receiver 28,30. A wavelengthselective switch 45 passes the probe wavelength λ2 to the probe receiver28,30. The probe/pilot channel is directed into the probe receiver 28,30where the electrical field (phase and amplitude) of the received fieldin the x and y polarization states (isolated by a polarization beamsplitter 46) is measured by an optical hybrid 48 with respect to thefield of a local oscillator 49 and converted by photodetectors 50 toelectrical signals. The signals are then digitized with analog todigital converters 52 and stored on a computer (for example, a digitalsignal processor 54) for processing.

An example of the processing of the resulting signals is as follows:

-   -   a. Load captured waveform    -   b. Remove chromatic dispersion (in some cases)    -   c. Remove intermodulation frequency between known transmit        waveform and received waveform by finding the time delay and        intermodulation frequency that maximizes the cross correlation        between the transmit and received waveforms.    -   d. Locate each 3p trial within the received record through        cross-correlation with the transmit waveform.    -   e. From each 3p trial extract the pump and probe fields |p(t)        and |r(t)        , respectively.

In some implementations, the computer that processes the storedwaveforms is a general purpose computer specially configured to processthe stored waveforms as described above. FIG. 13 is a simplifiedblock-diagram illustration of a general-purpose computer device 60. Aphysical, non-transitory computer-readable medium, such as a memory 62,stores computer-readable instructions 64, which when executed by aprocessor 66, result in the processing of stored waveforms 68 asdiscussed above. The stored waveforms 68 are illustrated as stored inthe memory 62, but may be stored in a different memory (not shown) ofthe general-purpose computer device 60. Various input/output components70 are coupled to the processor 66 to enable receipt of the waveforms68, control of various processing parameters, and output of the resultsof processing the waveforms 68.

Pulse Sequence and Shape Design

As described in the section entitled PUMP/PROBE/PILOT (3P) TRIALS, ameasurement sequence consists of pump pulses, probe pulses, and pilotpulses prepared in one or more states where properties such as therelative time delay, dispersion, amplitude, duration or polarizationstate are varied.

Repeated measurements with varying pump-probe-pilot configurationsenable one or more of the following properties of a bi-directionaloptical communication system 2,3 to be determined: 1)wavelength-dependent power profile and gain evolution along the opticallink 8; 2) wavelength-dependent dispersion map; and 3) location ofregions of high polarization-dependent loss (PDL) and polarization-modedispersion (PMD).

Traditional coherent optical time domain reflectometry (C-OTDR) is basedon monitoring the back scattered light as the probe pulse propagatesthrough the optical fiber. The measurement interval is given by theround trip time through the optical link, and the receiver mustcontinuously acquire during this time. In contrast, the techniquesdiscussed in this disclosure enable multiple pulses to be launched intothe optical link 8 at the same time, provided that no pump/probe/pilotmember from one trial spatially overlaps with any pump/probe/pilotmember from any other trial at any position along the link betweenlocations where the probe/pilot pulses are sampled and compared.

The minimum delay between pulses is roughly given by Equation (1), wherethe cumulative dispersion at the end of the optical link 8 and thewavelength separation Δλ are used to estimate the maximum walk-off. Forthe example of a 10,000 km of NDSF link (having a fiber dispersionparameter D of approximately 17 ps/nm/km) with a 2 nm wavelengthseparation between the pump pulses and the probe pulses, the minimumdelay between measurements is 0.3 μs.

For measurements employing HLLBs probe/pilot pulse replicas will bereflected back from each optical amplifier site and the pulse sequencemust ensure that replicas do not overlap on the return path. Forexample, for 10,000 km of NDSF (having a fiber dispersion parameter D ofapproximately 17 ps/nm/km) with a 2 nm wavelength separation between thepump pulses and the probe pulses, replicas of a probe pulse will bereturned every 0.8 ms, corresponding to the round trip time for a span10 of length 80 km. Additional measurements can be inserted between thereplicas provided that the pulses are separated by at least 0.3 μs.Within a 0.8 ms burst, 2667 pulse sequences (consisting of the pumppulse, the probe pulse and the pilot pulse) can be launched without riskof collision. Pulses will be detected at the receiver in 0.3 μsintervals until the burst has completed the 100 ms (2×10,000 km) roundtrip through the optical link 8, at which time the next burst of 2667pulses can be sent. With this sequence, a given span 10 is probed at26.6 kHz and the 125 spans 10 in the 10,000 km optical link 8 are probedsimultaneously for a total measurement acquisition rate of 3.3 MHz.

In a typical application the pulse sequence may be chosen to measure theproperties of a channel that would be experienced by a modulatedwaveform carrying traffic on that channel. As such the average power andpolarization state of the pump waveform may be chosen to match theproperties of the traffic-carrying waveform. The pump pulse, probe pulseand pilot pulses are surrounded by guard regions that insure that theprobe pulse spatially overlaps only with the pump pulse and that theprobe/pilot pulses do not spatially overlap with any other modulatedpart of the pump waveform at any position along the optical link.Outside of the guard region the pump waveform and the probe/pilotwaveforms are typically filled with a padding region consisting ofpolarization multiplexed phased modulated data symbols such asX-constellation symbols. See, for example, A. D. Shiner et al.,“Demonstration of an 8-dimensional modulation format with reducedinter-channel nonlinearities in a polarization multiplexed coherentsystem”, Opt. Express 22 (17), p.20366 (2014). The padding region helpsto present the amplifiers with a polarization diverse waveform andreduces the peak to root mean square (RMS) power ratio of the waveform.In most cases the duration of the pump waveform or the probe/pilotwaveform including the guard region is on the order of 100 nanosecondsto one microsecond. This timescale is much faster than the lifetime ofthe excited state of Er⁺ used in optical amplifiers and much faster thanthe response time of any control loops which may be running on theoptical amplifiers 12. Given the disparate timescales, the amplifierresponse will be governed by the average power and polarization of thepump waveform and the probe/pilot waveforms, and measurements forproperties such as power and PDL will not be perturbed by the dynamicresponse of the channel.

The optimal choice of pulse shape and duration depends on the wavelengthseparation Δλ between the pump pulses and the probe pulses, the pulsebandwidth and the dispersion. These properties determine the XPM andXPolM bandwidths which moderate the strength of the nonlinearinteraction between the pump channel and the probe channel. In effect,the magnitude of the phase shift, and with it the sensitivity of themeasurement, is inversely proportional to the spatial resolution. Withinreason, the durations of the pump pulse and the probe pulse can bechosen as convenient to generate, and then the separation between thechannels can be selected for the best trade-off between spatialresolution and sensitivity (averaging time).

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practised otherwise than as specifically described herein.

The foregoing discussion describes transmitting for each pump pulse onlya single probe pulse (and, possibly, one or more pilot pulses). In otherimplementations, multiple probe pulses may be used, each with adifferent initial time delay At relative to the pump pulse. For example,multiple probe pulses may be transmitted in the time between pilotpulses. If 20 probe pulses are squeezed in between two pilot pulses,then for each 3p trial, the nonlinear interaction is measured at 20different overlap interaction positions.

The common mode phase shift and or change in polarization state inducedin a probe pulse through interaction with a pump pulse is measured withrespect to a pilot pulse. In the ideal circumstances described abovethere is no interaction between the pump pulse and the pilot pulseduring propagation through the link. This ideal arrangement that thepilot pulse does not spatially overlap with any of the pump pulses maybe achieved by appropriate selection of the time delay between the pilotpulse and the pump pulse and appropriate pulse shaping. In practice,however, viable common mode phase and polarization state measurementsremain possible in cases where there is some interaction between thepump and pilot pulses provided that the error introduced through such aninteraction is small compared to the quantity being measured or forcases where the error is deterministic and can be corrected. Forexample, measurements remain viable for cases where the error inmeasured common mode phase shift is <50% or the error in measuredpolarization state is <90 degrees on the Poincaré sphere. Accordingly,in the claims, the phrase “pilot pulses arranged to not be spatiallyoverlapped in the optical fiber with any of the pump pulses” is intendedto include such cases where there is some interaction between the pilotpulses and the pump pulses that introduces a small error or adeterministic-yet-correctable error.

What is claimed is:
 1. A method for measuring physical properties ofoptical signals as a function of wavelength and as a function oflocation in an optical link, the method comprising: generating a firstmodulated optical carrier at a first wavelength, the first modulatedoptical carrier carrying pump pulses; generating a second modulatedoptical carrier at a second wavelength that differs from the firstwavelength, the second modulated optical carrier carrying pilot pulsesand carrying probe pulses; transmitting the first modulated opticalcarrier and the second modulated optical carrier on an optical fiberover the optical link, the optical link comprising multiple spansconnected by one or more optical amplifiers, such that a probe pulse ofthe probe pulses is spatially overlapped in the optical fiber with apump pulse of the pump pulses within at least one interaction region inthe optical link; performing a measurement on the probe pulse beyond anend of the optical fiber to measure optical properties of the probepulse relative to a coherent reference, wherein the coherent referenceincludes a component of at least one of the pilot pulses, the componentbeing coherent to the probe pulse; and calculating physical propertiesof the pump pulse from the measurement on the probe pulse, wherein thepilot pulses are arranged to be not spatially overlapped in the opticalfiber with any of the pump pulses, or the pilot pulses are arranged sothat any interaction between the pilot pulses and any of the pump pulsesintroduces a negligible error or a deterministic-yet-correctable errorin measurement of the optical properties.
 2. The method as recited inclaim 1, wherein the coherent reference includes a coherent component ofan advance pilot pulse of the pilot pulses in advance of the probe pulseand a coherent component of a following pilot pulse of the pilot pulsesfollowing the probe pulse, and wherein performing the measurement on theprobe pulse beyond the end of the optical fiber to measure opticalproperties of the probe pulse relative to the coherent referencecomprises using the advance pilot pulse and the following pilot pulse toestimate and correct differences between transmit and receive lasersources.
 3. The method as recited in claim 1, further comprisingadjusting a time delay between the pump pulse and the probe pulse. 4.The method as recited in claim 1, wherein the calculated physicalproperties of the pump pulse include a polarization state of the pumppulse within a particular span of the optical link or within theinteraction region.
 5. The method as recited in claim 1, furthercomprising determining a cumulative dispersion within each span alongthe optical link from the calculated physical properties of the pumppulse.
 6. The method as recited in claim 1, wherein the calculatedphysical properties of the pump pulse include a power of the pump pulseat the first wavelength at the one or more interaction regions.
 7. Themethod as recited in claim 1, wherein generating the first modulatedoptical carrier comprises modulating an in-service channel at the firstwavelength to carry the pump pulses.
 8. The method as recited in claim1, further comprising pre-distorting the pump pulses to have a desiredtemporal dependence at the one or more interaction regions.
 9. Themethod as recited in claim 1, further comprising repeating the methodwith the pump pulse in varying polarization states or with the probepulse in varying polarization states, or with both the pump pulse andthe probe pulse in varying polarization states.
 10. The method asrecited in claim 9, further comprising averaging the measurement on theprobe pulse for the varying polarization states to obtain an averagedmeasurement, wherein calculating physical properties of the pump pulsefrom the measurement on the probe pulse comprises calculating thephysical properties of the pump pulse from the averaged measurement. 11.The method as recited in claim 1, further comprising repeating themethod with the probe pulse in varying non-collinear polarizationstates, and the calculated physical properties of the pump pulse includea polarization state of the pump pulse within a particular span of theoptical link.
 12. The method as recited in claim 1, further comprisingrepeating the method with different separations between the firstwavelength and the second wavelength and with different interactionregions, and estimating a polarization mode dispersion (PMD) of theoptical link from the calculated physical properties of the pump pulse.13. The method as recited in claim 1, further comprising repeating themethod with the pump pulse in varying polarization states, determining afirst polarization state that maximizes the common mode phase and asecond polarization state that minimizes the common mode phase, andestimating an accumulated polarization dependent loss at the interactionregion as the difference between the first polarization state and thesecond polarization state.
 14. A monitoring system for an optical link,the monitoring system comprising: a first transmitter to generate afirst modulated optical carrier at a first wavelength, the firstmodulated optical carrier carrying pump pulses; a second transmitter togenerate a second modulated optical carrier at a second wavelength thatdiffers from the first wavelength, the second modulated optical carriercarrying pilot pulses and carrying probe pulses; the first transmitterto transmit the first modulated optical carrier on an optical fiber overthe optical link and the second transmitter to transmit the secondmodulated optical carrier on the optical fiber over the optical link,the optical link comprising multiple spans connected by one or moreoptical amplifiers, such that a probe pulse of the probe pulses isspatially overlapped in the optical fiber with a pump pulse of the pumppulses within at least one interaction region in the optical link; acoherent receiver to perform a measurement on the probe pulse beyond anend of the optical fiber to measure optical properties of the probepulse relative to a coherent reference, wherein the coherent referenceincludes a component of at least one of the pilot pulses, the componentbeing coherent to the probe pulse; and a processor to calculate physicalproperties of the pump pulse from the measurement on the probe pulse,wherein the pilot pulses are arranged to be not spatially overlapped inthe optical fiber with any of the pump pulses, or the pilot pulses arearranged so that any interaction between the pilot pulses and any of thepump pulses introduces a negligible error or adeterministic-yet-correctable error in measurement of the opticalproperties.
 15. The monitoring system as recited in claim 14, whereinthe calculated physical properties of the pump pulse include a power ofthe pump pulse at the first wavelength at the one or more interactionregions.
 16. The monitoring system as recited in claim 14, wherein thefirst transmitter is to generate the first modulated optical carrier bymodulating an in-service channel at the first wavelength to carry thepump pulses.
 17. The monitoring system as recited in claim 14, whereinthe first transmitter is a first coherent transmitter, the secondtransmitter is a second coherent transmitter, and the second coherenttransmitter is synchronized to the first coherent transmitter.
 18. Themonitoring system as recited in claim 14, wherein the coherent referenceincludes a coherent component of an advance pilot pulse of the pilotpulses in advance of the probe pulse and a coherent component of afollowing pilot pulse of the pilot pulses following the probe pulse, andwherein the coherent receiver is operative to use the advance pilotpulse and the following pilot pulse to estimate and correct differencesbetween transmit and receive laser sources.